Use sortedmulti() descriptor for BIP67

[Sjors]

Depends on Bitcoin Core master (bitcoin/bitcoin#17056)

ColdCard simulator no long complains about BIP67 violation (at least for the one change address that it complained about before).

[stepansnigirev]

What worries me is that sortedmulti will break compatibility with current and next release of Bitcoin Core. If I understand correctly 0.19 is in RC already and sortedmulti is scheduled for 0.20 release.
I see two ways to fix it at the moment:

• Generate addresses in python, add them to Bitcoin Core one by one if Core version doesn't support sortedmulti.
• Ignore current Core release, merge this pull request and write in the Readme.md that we only support master branch of Core at the moment.

What way should we take?

[Sjors]

sortedmulti is indeed scheduled for 0.20 and it's probably too late to add it to 0.19.

Given that this project is still experimental, I think the best approach is to use Bitcoin Core master, and then switch to 0.20 early next year. Hopefully native descriptor wallets also make it into that release, so you can just use getnewaddress.

[stepansnigirev]

Fixed with #22
I made a temporary workaround to import sortedmulti addresses in current BitcoinCore release. It sorts the xpubs for every index.